1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind which controls the air-fuel ratio of a mixture supplied to the engine in a feedback manner, by applying an adaptive control theory thereto.
2. Prior Art
Conventionally, there has been proposed an air-fuel ratio control system for internal combustion engines based on an adaptive control theory, e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 7-247886, which calculates an adaptive control correction coefficient in response to an output from an air-fuel ratio sensor arranged in the exhaust system of the engine by the use of an adaptive controller provided with a parameter-adjusting mechanism of a recurrence formula type, to thereby correct the amount of fuel supplied to the engine to feedback-control the air-fuel ratio of the mixture supplied to the engine.
However, in the conventional air-fuel ratio control system, the adaptive control correction coefficient is commonly used for all the cylinders, so that it is impossible to carry out the air-fuel ratio feedback control in a manner adapted to variations in characteristics between the cylinders. If the adaptive control correction coefficient is set cylinder by cylinder to overcome the above inconvenience, however, due to the fact that an output signal from the air-fuel ratio sensor which is arranged at a confluent portion of the exhaust system of the engine is input to the parameter-adjusting mechanism for adjustment of adaptive parameters, there arises the following problem:
Individual cylinders of the engine to be controlled by the air-fuel ratio control system are different from each other in operating characteristics in a strict sense, and therefore, when the adaptive parameters are adjusted based on the output signal from the air-fuel ratio sensor, the variations in the operating characteristics between the cylinders are not appropriately reflected in the adaptive parameters. Therefore, the conventional air-fuel ratio control system remains to be improved in respect of the controllability of the air-fuel ratio.